Use of a steel alloy

ABSTRACT

A steel grade for use in a hot forming and press hardening process has the following composition, in weight percent: C 0.15%=C&lt;0.35%, Mn 0.8-2.5%, Si 1.5-2.5%, Cr max. 0.4%, Al max. 0.1%, Ni max. 0.3%, B 0.0008-0.005%, Ti 0.005-0.1%, Nb max. 0.1%, remainder iron and unavoidable impurities as well as a hot formed and press hardened structural part made of this steel grade.

The invention describes the use of a steel alloy.

DE 24 52 486 C2 discloses a process for press forming and hardening asteel sheet of slight material thickness and good dimensional precision,whereby a steel sheet from a boron-alloyed steel is heated to above Ac₃and then pressed at substantial change in shape in less than 5 secondsinto the final shape between two indirectly cooled molds and subjectedto a rapid cooling while remaining in the press so as to obtain amartensitic and/or bainitic fine-grained structure. This process is tobe understood hereinafter as hot forming and press hardening and hasproven effective for producing high-strength, relatively thin parts ofcomplex configuration and high dimensional precision for structural andsafety parts such as A and B pillars or bumpers in the automobileindustry. Metal sheets of typical thicknesses of 3 mm or less are herebyformed and steels with slight carbon content are used. The mentionedprinted publication describes a steel alloy with C<0.4%; silicon of acontent dependent on the steel production process but essentiallynegligible; 0.5 to 2.0% Mn; max. 0.05% P; max. 0.05% S; 0.1 to 0.5% Cr,and/or 0.05 to 0.5% Mo; up to 0.1% Ti; 0.0005 to 0.01% B; up to a totalof 0.1% Al, and optional contents of copper and nickel up to 0.2% each.

A typical boron-alloyed steel for hot forming and hardening is disclosedfor example in DE 197 43 802 C2. DE 197 43 802 C2 describes a processfor the manufacture of a metallic molded structural part for motorvehicle components with regions of higher ductility. For this purpose, ablank is made of a steel alloy, which, expressed in weight percent,includes carbon (C) 0.18% to 0.3%; silicon (Si) 0.1% to 0.7%; manganese(Mn) 1.0% to 2.5%; phosphorous (P) at most 0.025%; chromium (Cr) 0.1% to0.8%; molybdenum (Mo) 0.1% to 0.5%; sulfur (S) at most 0.01%; titanium(Ti) 0.02% to 0.05%; boron (B) 0.002% to 0.005%; aluminum (Al) 0.01% to0.06% and remainder iron, including impurities resulting from smelting.The mentioned alloy is eminently suited for hot forming and presshardening. However, the alloying structure is predominantly composed ofmartensite when hardened. As a consequence, ductility is not alwayssufficient in the material for the respective load situation at hand.

DE 10 2005 054 847 B3 proposes therefore to use a hot formed and presshardened structural component which has been heat-treated at 320 to 400degrees Celsius after the hot forming and press hardening process. Thisheat treatment influences in a desired manner the high-strengthproperties of the component. The yield strength R_(p0.2) and theelongation A₅ remain nearly unchanged. Only the tensile strength valuesRm are reduced by 100 to 200 N/m². When the afore-described steel gradecomprised, in weight percent, of carbon (C) 0.18% to 0.3%, silicon (Si)0.1% to 0.7%, manganese (Mn) 1.0% to 2.5%, phosphorus (P) at most0.025%, chromium (Cr) up to 0.8%, molybdenum (Mo) up to 0.5%, sulfur (S)at most 0.01%, titanium (Ti) 0.02% to 0.05%, boron (B) 0.002% to 0.005%,and aluminum (Al) 0.01% to 0.06%, remainder iron including impuritiesresulting from smelting, is involved, a heat treatment at 320 to 400° C.results in a tensile strength Rm of 1200 to 1400 N/mm², a yield strengthR_(p0.2) of 950 to 1250 N/mm², and an elongation A₅ of 6-12%. Thematerial still has the necessary high-strength mechanical properties,but, due to the somewhat lower tensile strength Rm, it has enoughductility that it crumples instead of breaking or rupturing in acollision. The additional tempering process is however again relativelycomplicated and expensive.

U.S. Pat. No. 6,544,345 also belongs to the prior art and relates to theproduction of a high-strength steel alloy. The steel alloy has amicrostructure comprised of ferrite and/or bainite as well as residualaustenite. This steel alloy is especially suitable to absorb high forceswhen exposed to dynamic stress. The hot formed microstructure issuitable for press forming.

Further belonging to the state of the art are EP 2 003 221 A1 and EP 2039 791 A1 which relate each to high-strength steel alloys andrespective manufacturing methods.

In addition, so-called TRIP steels (engl. TRansformation InducedPlasticity, germ.: umwandlungsbewirkte Plastizität) are generally known.Especially high-strength steel alloys are involved here having amultiphase microstructure. TRIP steels are stronger while at the sametime more ductile than conventional steel grades. As a result, theyallow the manufacture of lighter structural parts at a predefinedrequired strength and elongation. The TRIP effect involves theparticular martensitic formation during shaping. This causes asimultaneous increase in hardness and formability at mechanicalformation in the product manufacture or product use. The realization ofthe effect is mainly influenced by the cost-efficient alloying elementsaluminum and silicon. In addition, significantly more expensive alloyingelements such as nickel can be saved. The material-inherent yieldstrength is higher than comparable steels because silicon permits theformation of solid solution strengthening. As soon as the deformationreaches the plastic range, the metastable carbon-rich austenite begins astrain-induced transformation into martensite. As a result, the strengthof the TRIP steel is tailored after plastic deformation. Cold formedstructural parts with high yield strength and tensile strength arehowever limited as far as complexity of the geometry is concerned.Moreover, cold forming requires consideration of an elastic recovery ofthe steel, when the tool is designed. In addition, the residualelongation in the formed region is lower than in the region that is notformed. The structural part has therefore uneven properties.

WO 2004/022794 A1 shows a method for the production of a steel with aproportion of residual austenite in the steel microstructure, whereby arespective steel is heated to produce austenite and then quenched totransform at least in part the austenite into martensite. Then, carbonis partitioned to transfer carbon from the martensite to the stillpresent austenite. This partitioning takes place in the range of themartensitic starting temperature. Therefore, the steel is held in thistemperature range for a respectively long period or heated again andthen cooled down in a desired manner. WO 2004/022794 A1 does notdisclose a boron-alloyed steel.

DE 10 2008 010 168 A1 discloses the use of a steel grade for armoring avehicle, having a composition, expressed in weight percent, of 0.35 to0.55% of carbon; 0.1 to 2.5% of silicon; 0.3 to 2.5% of manganese; max.0.05% of phosphorus; max. 0.01% of sulfur; max. 0.08% of aluminum; max.0.5% of copper; 0.1 to 2.0% of chromium; max. 3.0% of nickel; max. 1.0%of molybdenum; max. 2.0% of cobalt; 0.001 to 0.005% of boron; 0.01 to0.08 of niobium; max. 0.4% of vanadium; max. 0.02% nitrogen; max. 0.2%titanium; remainder iron including impurities resulting from smelting.Also this steel grade is hot formed. Apart from the application of thisalloy for armoring purposes, it has a relatively high carbon contentwhich decreases welding capability.

Starting from this state of the art, the invention is therefore based onthe object to provide a hot formed and press hardened structural partwith a high yield strength and a high tensile strength but at the sametime has a better ductility compared to the state of the art.

This object is solved by the use of a steel alloy which has acomposition, expressed in weight percent, of:

C 0.15% ≦ C < 0.35% Mn 0.8-2.5% Si 1.5-2.5% Cr max. 0.4% Al max. 0.1% Nimax. 0.3% B 0.0008-0.005% Ti 0.005-0.1% Nb max. 0.1%,remainder iron and unavoidable impurities in a hot forming and presshardening process. A blank separated from a strip material or apreviously pre-formed structural part is heated hereby to a temperatureabove the Ac₃ point of the alloy so as to transform the microstructureinto austenite. Thereafter, the blank or the preformed structural partis placed in a force-cooled mold, shaped and hardened at the same timeby cooling it to a temperature below approximately 200° C. Pressing inthe closed mold prevents distortion. Subsequently, the finished hotformed and press hardened structural part is removed from the mold. Theparticular composition of the steel, especially the relativelysubstantial addition of silicon, not only produces martensite duringhardening. Instead, part of austenite is retained as residual austenitewhich remains stable up to temperatures of minus 100° C. Themicrostructure may contain also proportions of bainite in addition toresidual austenite. The silicon in the steel prevents carbide formationso that carbon is available for stabilizing the residual austenite. Theresidual austenite provides the steel according to the invention with ahigher breaking elongation than classic boron-alloyed, purelymartensitic hot formed steel. Moreover, a later deformation, e.g. in theevent of a crash when structural or safety components are involved andfor which the hot formed and press hardened structural parts aretypically used, causes formation of martensite from the still presentresidual austenite so that the steel is additionally hardened in theevent of a crash. As a result, tensile strengths are attained which arecomparable to the conventional hot formed steel with comparable carboncontent.

The desired microstructure is reached by the invention not in the hotrolling process but during the hot forming process (press hardening).When the microstructure is present already after hot rolling, the steelis suitable for cold forming. When forming the steel, the metastableresidual austenite, present in the hot strip, can be transformed intomartensite. On the other hand, during hot forming/press hardening, thehot strip, which may have any microstructure in the initial state, isaustenitized, hot formed and press hardened so that in combination witha following tempering the desired microstructure of predominantlymartensite with proportions of bainite and residual austenite isrealized.

According to a preferred embodiment, the steel according to theinvention has the following composition, expressed in weight percent,of:

C 0.22-0.25% Mn 1.5-1.7% Si 1.95-2.1% Cr max. 0.15% Al 0.03-0.05% Nimax. 0.2% B 0.002-0.0035% P max. 0.015% S max. 0.01%, Ti 0.005-0.1% Nbmax. 0.1% N max. 0.01%,remainder iron and unavoidable impurities. Preferably is hereby theratio of titanium to nitrogen 1 Ti to 3.4 N to 5 N. In this way, enoughnitrogen is bound by titanium. After heating above Ac₃ and hot formingand press hardening in a hot forming tool which is indirectly cooledwith water, this alloy composition reaches a yield strength Rm of >1600MPa N/mm², a tensile strength R_(p0.2) of >1050 MPa, and a breakingelongation A₅ of >10.5%. The hardened microstructure is comprised ofmartensite and residual austenite.

As a result of the proportions of residual austenite in the finishedstructural part, the breaking elongation of the structural part isincreased. A fastest possible and direct cooling process that is typicalfor press hardening is sufficient for reaching the desiredmicrostructure. There is no need for a separate carbon partitioning. Anelastic recovery of the material is not to be expected as a result ofthe hot forming and press hardening. Moreover, in view of the highproportion of silicon, the surface oxidizes less than in conventionalhot formed steels. As a consequence, it is possible to produce a hotformed and press hardened structural part with a surface which can becoated directly with EDP in the absence of a preceding irradiation.Moreover, the hardened steel is more stable for tempering as a result ofthe high proportion of silicon. The development of carbides duringtempering is suppressed so that the material can be galvanized even at400 to 450° C., while the tensile strength Rm remains still >1450 MPa.As the high silicon content increases the Ac₃ temperature of the alloy,the heating temperature must be selected respectively higher. It is atleast 960° C., when the silicon content is 2%.

Overall, the use in accordance with the invention of the alloycomposition according to the invention in a hot forming and presshardening process is well suited to produce a dimensionally precisehigh-strength structural part with increased ductility.

1.-8. (canceled)
 9. A steel alloy for use in a hot forming and presshardening process, said steel alloy comprising in weight percent: C0.15% = C < 0.35% Mn 0.8-2.5% Si 1.5-2.5% Cr max. 0.4% Al max. 0.1% Nimax. 0.3% B 0.0008-0.005% Ti 0.005-0.1% Nb max. 0.1%,

remainder iron and unavoidable impurities.
 10. The steel alloy of claim9, comprising in weight percent: C 0.22-0.25 % Mn 1.5-1.7% Si 1.95-2.1%Cr max. 0.15% Al 0.03-0.05% Ni max. 0.2 % B 0.002-0.0035% P max. 0.015%S max. 0.01%, Ti 0.005-0.1% Nb max. 0.1% N max. 0.01%,

remainder iron and unavoidable impurities.
 11. The steel alloy of claim9, wherein a ratio of Ti to N is 1:3.4 to
 5. 12. The steel alloy ofclaim 9, wherein the steel alloy has a microstructure comprisedpredominantly of martensite with proportions of residual austenite andbainite.
 13. A hot formed and press hardened structural part made from asteel grade comprising, in weight percent: C 0.15% = C < 0.35% Mn0.8-2.5 % Si 1.5-2.5 % Cr max. 0.4% Al max. 0.1% Ni max. 0.3% B0.0008-0.005% Ti 0.005-0.1% Nb max. 0.1 %,

remainder iron and unavoidable impurities.
 14. The hot formed and presshardened structural part of claim 13, wherein the steel grade is made ofa composition, in weight percent of: C 0.22-0.25% Mn 1.5-1.7% Si1.95-2.1% Cr max. 0.15% Al 0.03-0.05% Ni max. 0.2% B 0.002-0.0035% Pmax. 0.015% S max. 0.01%, Ti 0.005-0.1% Nb max. 0.1% N max. 0.01%,

remainder iron and unavoidable impurities
 15. The hot formed and presshardened structural part of claim 13, wherein the steel grade haspredominantly a martensitic microstructure with proportions of residualaustenite and bainite.
 16. The hot formed and press hardened structuralpart of claim 13, for use as a structural and/or safety component.
 17. Amethod of making a steel alloy, comprising the steps of: heating a blankhaving a steel composition comprising in weight percent: C 0.15% = C <0.35% Mn 0.8-2.5% Si 1.5-2.5% Cr max. 0.4% Al max. 0.1% Ni max. 0.3% B0.0008-0.005% Ti 0.005-0.1% Nb max. 0.1%,

remainder iron and unavoidable impurities, to a temperature above Ac₃;placing the blank in a force-cooled mold; hot forming and presshardening the blank; and cooling the blank to a temperature below 200°C.
 18. The method of claim 17, wherein the hot forming and presshardening step is carried out, while the mold is closed.
 19. The methodof claim 17, wherein the steel composition comprises, in weight percent:C 0.22-0.25% Mn 1.5-1.7% Si 1.95-2.1% Cr max. 0.15% Al 0.03-0.05% Nimax. 0.2% B 0.002-0.0035% P max. 0.015% S max. 0.01%, Ti 0.005-0.1% Nbmax. 0.1% N max. 0.01%,

remainder iron and unavoidable impurities.